1. Field of the Invention
This invention relates to a method and apparatus for automatically testing, monitoring and maintaining an aircraft battery system and more particularly to a system by which the condition of the aircraft storage battery, as well as the low temperature starting limit, maintenance requirements and service life thereof can be monitored.
2. Description of the Prior Art
Systems for determining one or two of such battery conditions as battery capacity, state of charge, defects in the battery and the charging system, maintenance requirements and timely indication of the need to replace the battery or the on board charger, have been developed to facilitate maintenance of a reliable and safe aircraft battery system. Manual determination of these parameters is expensive, and time consuming. With the introduction of microprocessors, the battery characteristics, engine starting requirements and operational characteristics can be determined automatically, providing optimum life and safety and reliability.
Aircraft batteries provide power for starting, prestart check-ups, communications with control tower and canopy operation. Of these, the starting function is the first one affected by a weakened battery because the starter needs more power than other components. Since the above functions are vital for the reliable and safe operation of the aircrafts, it is useful to know when the battery is on the verge of losing its useful life.
Aircrafts have been using nickel cadmium batteries because of their light weight. However, the maintainance of these batteries has been a primary concern. Therefore, recently sealed lead acid batteries are being introduced for aircraft application. Any device that advises the user about the maintainance requirements, state of charge, battery health, and the end of useful life of these aircraft lead acid batteries will be very useful.
Generally the nominal voltage of these batteries is 28 volts. The battery capacity can be from 7 Ah to several tens of amperehours. These batteries are very similar to the SLI batteries in their function. The battery gives out large current pulse to start the engine and almost immediately after starting an on-board charger recharges the battery. Some aircrafts are started by other means such as compressed air capsules.
Several conditions of component status can cause aircraft battery systems failure. These are: Low state of charge of the battery, loss of the battery capacity, corroded terminals, low level of the electrolyte defective charging system and defective starter motor. A battery status monitor should function to sort out there problems automatically. The monitor should diagnose a problem as it develops and warn the user in advance of imminent failure. However, battery monitors heretofore disclosed do not identify each of the aforementioned problems; most of them attempt to determine only the state of charge of the battery.
Battery monitors originally comprised merely an ammeter and/or a voltmeter. They provided limited information of a type useful only to knowledgeable users. To have the battery tested, the battery was disconnected from the electrical system by a skilled mechanic and discharged through a known load. Alternatively, monitoring instruments were employed to derive the battery status parameters using an a.c. signal without discharging the battery through a load. Such monitoring instruments require that the battery be disconnected from the electrical circuitry of the user system. Even then they provide information about the state of charge solely.
Conventionally, the state of charge of the battery has been determined by measuring specific gravity of the electrolyte. This method is predicated on the fact that the specific gravity of the sulfuric acid electrolyte decreases when the battery is discharged and increases when the battery is charged. Normally, the specific gravity of the battery electrolyte ranges from about 1.28-1.30 when the battery is fully charged. Such a method is laborious and cannot predict loss of battery capacity in advance. Furthermore, such problems as corroded terminals are not detected by this method.
An alternative method for determining the state of battery charge involves the continuous counting of charge into and out of the battery. This method is subject to accumulation of error due to the current consumed by gassing in the battery and changes in battery capacity arising from corrosion of the electrode plates and shedding of active material. A device of this type using a current to frequency converter, is described in U.S. Pat. No. 4,678,999 to J. F. Schneider.
A battery's maximum power output is determined by its voltage and internal impedance. Generally the internal impedance of the battery increases with the life of the battery and with a decrease in its state of charge. In U.S. Pat. No. 4,678,998 to Muramatsu, there is described a device based on this principle. According to Muramatsu, the relationship between battery impedance, remaining capacity and remaining service life is predetermined at different frequencies and stored in memory. To determine the battery condition, a computer evaluates the internal impedance at a few frequencies and looks up to the stored table of predetermined values to obtain the remaining service life of the battery. Such a device is subject to the drawback that each battery or battery group has to be subjected to tests before installation to generate the table of predetermined values.
U.S. Pat. No. 4,665,370 to Holland describes a battery monitor wherein a cranking motor furnishes the load for the battery and an electronic apparatus automatically measures the condition of the battery each time the engine is started. The battery monitor compares the unloaded battery voltage with the voltage under load and indicates that the battery is bad if the voltage differential exceeds a predetermined limit. One disadvantage of this device is the inability thereof to distinguish between a low state of battery charge, a fully charged battery having low capacity and additional faulty conditions of the battery such as corroded terminals or low electrolyte level.